Okay I sound a bit like a two-year old (and probably sometimes behave like one as well!) but the question “why?” is often a much more important question than “what?” in microbiology and medicine. Let me explain…

​A 45 year old patient comes into hospital with a fever, back pain and numbness and tingling in his feet. Examination reveals tenderness over his L4/L5 vertebrae. There is a clinical suspicion that this patient might have a vertebral disciitis or osteomyelitis and he undergoes an MRI scan which confirms disciitis with surrounding oedema putting pressure on his spinal cord. Because he has neurological signs he is taken to theatre by the spinal surgeons who decompress the area and send samples of tissue (not a swab!!!) to the microbiology lab for culture. After the surgery he is started on IV Flucloxacillin as the most common bacterium to cause this type of infection is Staphylococcus aureus. The following day the laboratory grows a Streptococcus oralis sensitive to penicillin from the tissue samples.

The obvious question is “what should I treat the patient with now that I know the diagnosis and the causative bacterium?” That is the easy question to answer; logically the patient would be changed to the narrow spectrum treatment of IV Benzylpenicillin (you don’t treat disciitis with oral antibiotics). The intention would be to give them 6 weeks of treatment. But just doing this would be letting the patient down. There is a much more fundamental question to ask; “why has this patient got S. oralis in his back?” If we don’t answer the question “why?” then we may not prevent it happening again and we may miss a more serious underlying diagnosis that needs urgent treatment.

In order to answer the question “why” the first step is to consider why the bacterium got into the patients back, where did it come from and why did it get from there to somewhere it doesn’t belong. Only then can you decide what should be done to prevent this happening again.

How did the bacterium get into the patients back?There are only two ways a bacterium can get into a patients back; either it is directly introduced from outside by some form of penetrating injury or surgery, or it settled there after being in the blood stream. Once in the bloodstream the bacterium will be carried into any body site that the blood enters. If the patient has a chronically inflamed back due to osteoarthritis, or temporary inflammation due to minor trauma, then the bacterium will be carried in as the blood enters to try and deal with the tissue damage. As a result the bacterium can become stuck in the tissue and start to multiply causing an infection.

Haematogenous seeding like this is much more common than direct inoculation and so it is very likely that this patient has been bacteraemic with S. oralis at some time in the recent past. Since blood and vertebral tissue are normally sterile the S. oralis has to have come from somewhere else in the body, but where?

Where does S. oralis come from?S. oralis is one of the viridans streptococci and normally lives in the upper GI tract including the mouth (the name is a bit of a giveaway, oralis = mouth), but it can also occasionally be found in the lower GI tract. This bacterium is not normally invasive but does sometimes temporarily get into the blood stream after vigorous brushing of the teeth. This bacteraemia is short lived as the body’s immune system rapidly mops it up. It is possible that the patient has such poor dentition that they are bacteraemic much of the time but having looked in their mouth the dentition was good and therefore not a route in.

So if it’s not from the mouth, could it be from somewhere else in the upper GI tract? Attempts should be made to find the alternative source. The patient should be questioned about symptoms specific to the gastrointestinal tract such as abdominal pain, nausea, vomiting, jaundice, change in bowel habit etc. Further examination, to elicit any tenderness, may help to narrow down the focus of further investigations.

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​You’ll notice a bit of a trend in the above table. Carcinoma can cause bacteraemias even when there are few symptoms and signs to detect. This is because cancers disrupt the normal mucosal surfaces of body organs allowing the bacterial contents to “leak out” into the blood stream.

If there are no obvious pointers to the potential source then it is worth considering having a look for a potential problem. The most common ways of doing this are radiologically and endoscopically. A CT of the abdomen and an endoscopic examination of the upper (and possibly the lower) GI tracts is often warranted and may well identify an otherwise undiagnosed problem.

In this patient the CT abdomen was reported as normal but the endoscopy showed that something was pressing against the back of the stomach. A repeat CT, taking much more closely spaced imaging slices showed a small mass in the head of the pancreas behind the stomach.

What should be done to prevent this infection from occurring again?Once you know what microorganism is causing an infection, where it has come from, and why it has got from its normal body site to the infected area, then there can be an informed decision made about whether anything can and should be done to prevent it from occurring again. This is not always simple, but at least the patient and the doctors can now decide what to do.

Diagnosing cancer, at a stage when something relatively simple can still be done to cure the patient either through surgery or chemotherapy, based on a knowledge of bacteria is one of those astonishing things that microbiologists can spring on an unsuspecting ward round. It might be that a Dukes A or B bowel malignancy can be identified and removed surgically before it has time to metastasise, or it may be as in this patient that an early pancreatic cancer can be detected and excised successfully.

The table below shows some other possible “whys?” which illustrate the wide variation in potential underlying reasons for various clinical scenarios and what might be done to prevent the infections recurring. The list is not exhaustive as there are all sorts of possible reasons for why things might happen. The important thing to do is ask the question “why?”

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​So yes, I am often like a 2 year old asking the question “why?” It might be one of the most irritating questions anyone can ask, especially when the answer isn’t known, but in medicine it is one of the most important questions. Why did this happen? Why this patient? Only then can we answer the question “what can we do to prevent it happening again?”

Have you come across any good scenarios where the answer to the question “why?” made a difference to your patients? Let me know, I’m always on the lookout for good teaching scenarios…